The development of modern rubber materials has made possible the manufacture of a wide range of elastomeric articles having varying properties of strength and chemical resistance. One useful class of rubber compounds is the nitrile rubber class, which is widely used to make a variety of articles of manufacture.
Carboxylated nitrile, which is a terpolymer of butadiene, acrylonitrile, and organic acid monomers, has at least two properties that make it useful for manufacturing elastomeric articles. These two features are high strength and impermeability to certain hydrocarbon solvents and oils. Compounding and curing the rubber (which is used in latex form for, e.g., dipping to provide articles of manufacture such as gloves or condoms) with other ingredients such as curing agents, accelerators, and activators is generally performed to optimize these properties. The level of each monomer in the polymer and the level of curing affect the levels of strength and the chemical resistance in the finished article. Polymers with higher levels of acrylonitrile tend to have better resistance to aliphatic oils and solvents, but are also stiffer than polymers that have lower levels of acrylonitrile. While the chemical nature of the monomers from which the polymer is made offers some degree of chemical resistance, when the polymer molecules are chemically crosslinked, resistance to chemical swelling, permeation, and dissolution greatly increase.
Crosslinking also increases the strength and elasticity of the rubber. Carboxylated nitrile latexes can be chemically crosslinked in at least two ways: the butadiene subunits can be covalently crosslinked with sulfur/accelerator systems; and the carboxylated (organic acid) sites can be ionically crosslinked with metal oxides or salts. Sulfur crosslinks often result in large improvements in oil and chemical resistance. Ionic crosslinks, resulting from, for example, the addition of zinc oxide to the latex, result in a rubber having high tensile strength, puncture resistance, and abrasion resistance, as well as high elastic modulus (a measure of the force required to stretch a film of the rubber), but poor oil and chemical resistance. Many currently available rubber formulations generally employ a combination of the two curing mechanisms. For example, in combination with sulfur and accelerators, carboxylated nitrile latex manufacturers frequently recommend addition of 1-10 parts of zinc oxide per 100 parts of rubber.
When zinc oxide is not employed, the curing time required to reach an optimum state of cure can be much longer and the curing may be less efficient. This means that the crosslinks are longer (more sulfur atoms per crosslink) and there may be a higher amount of sulfur that does not crosslink polymer chains. The result can be a less-effectively cured rubber that has lowered heat resistance and less chemical resistance.
However, ionic crosslinking often increases the stiffness of an article made from the rubber. This is a disadvantage for applications in which a softer rubber is needed. For example, surgical gloves made of soft rubbers can provide greater tactile sensitivity for the wearer, which is desirable to improve the surgeon's "feel" during operations and to prevent fatigue of the hands.
A more comfortable nitrile glove that is easier to stretch, i.e. has lower elastic modulus, can be made using a polymer which contains less acrylonitrile or by crosslinking the polymer to a lesser degree. These changes, however, often compromise strength, chemical resistance, or both, resulting in articles that are unsuitable for many applications.
Accordingly, a soft rubber having strength and chemical resistance similar to stiffer rubbers is highly desirable.